An OPS-laser is increasingly being recognized as being a laser device that can provide a beam of continuous wave (CW) laser radiation of high brightness and high quality. An OPS-laser includes an OPS-structure typically comprising a multilayer mirror-structure surmounted by a semiconductor multilayer gain structure, with the layers epitaxially grown. The gain structure includes a plurality of active layers separated by barrier layers. The composition of the active layers primarily determines the lasing (peak-gain) wavelength.
In theory at least, there are semiconductor alloy material groups that can be used for such OPS-structures that will provide laser radiation at any wavelength from the ultraviolet to the mid infrared portions of the electromagnetic spectrum. In practice, only wavelengths in certain ranges can be generated with high power and reasonable efficiency. This can be due, inter alia, to lack of adequate conversion efficiency in a particular semiconductor alloy compositions, instability of an alloy, or lack of an adequate pump-light source.
By way of example blue-green and green wavelengths can in theory be generated in lasers made from II-VI semiconductor alloys. These must be pumped, however, by blue diode-lasers which at present have limited power output and efficiency. In practice, blue and blue-green wavelengths can be more efficiently and reliably generated by intra-cavity second-harmonic conversion (frequency-doubling) of the fundamental radiation of OPS-lasers made from III-V semiconductor alloys such as indium gallium arsenide phosphide (InGaAsP) which have relatively high conversion efficiency and can be pumped with radiation from III-V diode-lasers which also have high conversion efficiency.
There are laser applications that require several Watts (W) of high quality beam power in the orange and red regions of the visible spectrum. Red laser light having a wavelength of about 630 nanometers (nm) is useful in laser display applications. Orange and yellow laser light having a wavelength between about 570 nm and 590 nm is useful in certain laser skin-treatments. Unfortunately, in an OPS-laser system, none of these wavelengths can be efficiently generated with adequate power and efficiency as a fundamental wavelength beam, or by harmonic-conversion of a fundamental wavelength beam having a wavelength two or more times longer than the red, orange, or yellow wavelength.
Further, although an OPS-structure based on any given semiconductor alloy system may be configured to operate at any one wavelength within a range of wavelengths characteristic of that alloy system, the configuration may involve a relatively lengthy experimental effort. This is because a new configuration has to be optimized for minimizing stress or strain from the growth process, and to refine the composition of the quantum-well layers from a theoretical composition to provide the actual wavelength. If the volume of OPS-structures (chips) required is relatively small, the optimization effort can add significantly and even prohibitively to the unit cost.